Lipids can survive long geological intervals within sediments and provide a unique tool that allows the reconstruction of past organismic diversity and environmental conditions. The lipids discussed ...in this review include all substances produced by organisms that are insoluble in water but extractable by organic solvents. Lipid biomarkers refer to both functionalized biolipids as well as their hydrocarbon derivatives in geological materials that contain diverse information about biotic sources and environmental conditions. In spite of diagenetic and catagenetic alteration, lipid biomarkers commonly preserve the hydrocarbon structure of their biotic counterparts and have been found in rocks up to 1.6 billion years in age. These features have promoted the use of lipid biomarkers in many fields, including petroleum geology, paleoclimatology, oceanography, meteorology, geobiology and environmental science. Here, we (i) review the use of lipid biomarker records for the reconstruction of environmental conditions in deep time, including climatic conditions (temperature), sedimentary environments (redox, salinity and chemical composition) as well as catastrophic terrestrial events (soil erosion and wildfire), and (ii) generate new insights into environmental perturbations during the Permian-Triassic transition based on investigation of lipid biomarkers. We further propose that the ratio of dibenzothiophene to phenanthrene (DBT/P) in marine carbonates may be a robust proxy for seawater sulfate concentrations in deep time. Our compiled DBT/P records show substantial variations in seawater sulfate levels through Earth history that are consistent with the results of other proxies. We discuss the future outlook for application of lipid biomarker records to deep-time environmental research.
The Coniacian-Santonian (C-S) was a time of differentiation in marine sedimentation, characterized by organic carbon (OC)-rich black shales and carbonates interpreted as the last oceanic anoxic ...event, OAE3, versus OC-poor white/reddish limestones, chalk, and claystones known as Cretaceous Oceanic Red Beds (CORBs). Based on compiled geochemical and isotope proxy data of more than 95 study sites and sections, two high-resolution global carbon isotope curves for C-S carbonate and organic matter (OM) were reconstructed based on statistical analysis and discriminated three main levels of short amplitude (around 0.5‰), yet globally recognizable, carbon isotope excursions. These excursions, each some 0.4 to 0.7 Ma in duration, are characterized by regionally restricted benthic anoxia and sea-level highstands that best explain the OM accumulation during the OAE3 subevents defined herein as OAE3a (late mid-Coniacian, ca. 86.9 Ma, Kingsdown Event), OAE3b (late mid-Santonian, ca. 85.0 Ma, Horseshoe Bay Event), and OAE3c (late Santonian to Santonian-Campanian Boundary Event, ca. 83.5 Ma).
For a better understanding of the C-S climate evolution on a regional to global scale, a global compilation of δ18O from benthic and planktonic foraminifers and bulk carbonate was conducted and tested for pCO2 trends based on Δ13C curves. Thus, the C-S palaeoclimate can be divided into (1) a steady state phase of warm greenhouse climate during the Coniacian, followed by (2) a hot greenhouse during the early Santonian that might be consistent with the activation of the Central Kerguelen large igneous province (LIP), and (3) a longer-term cooling of the warm greenhouse climate from the mid-Santonian onwards. The mechanism controlling OC-poor versus OC-rich deposition can be attributed mainly to palaeoceanographic conditions such as water column oxygenation and circulation pattern changes during the C-S. OM-rich deposition is largely restricted to the low-latitude Atlantic and adjacent epeiric and shelf seas. Areas of enhanced oceanic circulation systems with a westward-directed Tethyan current, and regional eddies of water mass flow had negative feedback on OM accumulation and preservation during the C-S, which resulted in well-developed water column oxygen content. This, in turn, oxidized OM and led to deposition of OM-poor facies and CORBs in large parts of the Late Cretaceous oceans.
•Widespread anoxia was regionally restricted to the equatorial Atlantic and adjacent epicontinental seas.•Two high-resolution global δ13Ccarb and δ13Corg stacked curves for the C-S are presented.•We distinguished three short amplitude, yet globally recognizable, CIE defined as OAE3a, OAE3b, and OAE3c.•Global high-resolution δ18O curves showed three phases of climate state for the C-S.•Palaeoceanographic and circulation patterns played a role in OC-poor versus OC-rich deposition.
Authigenic carbonate (AC) forms in siliciclastic marine sediments in part as a result of the degradation of organic matter and methane. Degradation pathways vary considerably and each impact the ...chemical evolution of sediments and porewater differently. The relative importance of these reactions in contributing to carbonate authigenesis are poorly understood, especially from a global perspective. Modern porewater geochemical data and authigenic carbonate carbon isotope compositions (δ13Cac) of globally distributed marine sediment sites allow direct assessment of the relative importance of diagenetic pathways. Common correlations between bulk δ13Cac and depth reveal that AC tends to form progressively by multiple reaction pathways, rather than within a particular sediment horizon. A general lack of shallow AC within sediments 1) containing porewater sulfate and 2) exhibiting decreasing dissolved inorganic carbon (DIC) δ13C with depth suggest that organotrophic sulfate reduction does not promote significant authigenesis. Instead, the anaerobic oxidation of methane (AOM) may (in part) account for most AC expressing 13C-depleted isotope compositions, including those with δ13C values between −25 (the approximate marine organic matter value) and 0‰ VPDB. Widespread increases in δ13Cac with depth in conjunction with values that exceed seawater DIC compositions indicate precipitation in sediments exhibiting methanogenesis, likely aided by contemporaneous marine silicate weathering. Deeper AC formation may occur in sediments experiencing thermal decarboxylation. When data from all sites are considered collectively, sediments experiencing AOM and methanogenesis emerge as the most significant in yielding AC. The relative importance of authigenesis pathways controls potential impacts of AC deposition on marine carbon budgets and provides insight into the geochemical signatures exhibited by ancient carbonate concretions.
Benthic foraminiferal oxygen isotopic (δ18O) and carbon isotopic (δ13C) trends, constructed from compilations of data series from multiple ocean sites, provide one of the primary means of ...reconstructing changes in the ocean interior. These records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records. We present new benthic foraminiferal δ18O and δ13C compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to ∼80 Ma and tentatively to ∼110 Ma. First‐order variations in interbasinal isotopic gradients delineate transitions from interior ocean heterogeneity during the Late Cretaceous (>∼65 Ma) to early Paleogene (35–65 Ma) homogeneity and a return to heterogeneity in the late Paleogene–early Neogene (35–0 Ma). We propose that these transitions reflect alterations in a first‐order characteristic of ocean circulation: the ability of winds to make water in the deep ocean circulate. We document the initiation of large interbasinal δ18O gradients in the early Oligocene and link the variations in interbasinal δ18O gradients from the middle Eocene to Oligocene with the increasing influence of wind‐driven mixing due to the gradual tectonic opening of Southern Ocean passages and initiation and strengthening of the Antarctic Circumpolar Current. The role of wind‐driven upwelling, possibly associated with a Tethyan Circumequatorial Current, in controlling Late Cretaceous interior ocean heterogeneity should be the subject of further research.
The mid-Piacenzian is known as a period of relative warmth when compared to the present day. A comprehensive understanding of conditions during the Piacenzian serves as both a conceptual model and a ...source for boundary conditions as well as means of verification of global climate model experiments. In this paper we present the PRISM4 reconstruction, a paleoenvironmental reconstruction of the mid-Piacenzian (∼ 3 Ma) containing data for paleogeography, land and sea ice, sea-surface temperature, vegetation, soils, and lakes. Our retrodicted paleogeography takes into account glacial isostatic adjustments and changes in dynamic topography. Soils and lakes, both significant as land surface features, are introduced to the PRISM reconstruction for the first time. Sea-surface temperature and vegetation reconstructions are unchanged but now have confidence assessments. The PRISM4 reconstruction is being used as boundary condition data for the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) experiments.
The Atlantic is the only ocean basin almost entirely surrounded by passive margins, and a major global long‐term sink of carbonate carbon that has evaded subduction. Quantifying the history of ...carbonate accumulation in the Atlantic has been limited by the absence of well‐defined regional carbonate compensation depth (CCD) models. We determine the CCD for the northern North Atlantic, central North Atlantic, and South Atlantic, and use these reconstructions to compute the carbonate carbon mass and carbonate carbon flux in a tectonic framework at 0.5 m.y. intervals since 66 Ma. We find that the total carbonate carbon mass of the Atlantic has grown 2.5‐fold from ∼1,500 Mt at 66 Ma to ∼3,800 Mt at present day. The overall Cenozoic increase in carbonate carbon flux toward the present day is punctuated by “carbonate crash” phases in the mid‐Eocene at ∼44–38 Ma and in the mid‐late Miocene at ∼19–8 Ma. During these times the flux decreases from ∼45 to ∼25 Mt C/yr, likely caused by carbonate dissolution and reductions in productivity. Reduced carbonate carbon flux in the mid‐Eocene also coincides with reduced calcification rates of small coccolithophores previously observed offshore Africa. After ∼8 Ma the carbonate carbon flux rises to a Cenozoic maximum of ∼75 Mt C/yr at ∼3 Ma, possibly driven by enhanced flux of nutrients into the ocean. Our CCD curves and the resulting carbonate accumulation history are useful for calibrating ocean chemistry models, and constraining global terrestrial weathering rates, climate perturbations, and carbon cycle models.
Key Points
Carbonate carbon flux and storage has been computed for the Atlantic Ocean spanning the entire Cenozoic at 0.5 m.y. intervals
The total carbonate carbon mass of the Atlantic has grown 2.5‐fold from ∼1,500 Mt at 66 Ma to ∼3,800 Mt at present day
Carbonate carbon flux fluctuations are linked to carbonate crash and bloom phases, and changes in deep‐water circulation
The two major oceanic anoxic events of the Cretaceous, those of the Early Aptian (OAE 1a) and the Cenomanian-Turonian boundary (OAE 2), registered some of the highest temperatures reconstructed for ...the Cretaceous Period, and are thought to be related to the input of volcanically derived carbon dioxide from one or more Large Igneous Provinces. Widely distributed deposition of marine organic matter, the hallmark of OAEs, and intensified silicate weathering in response to a globally accelerated hydrological cycle and/or reaction of seawater with freshly extruded basalt, are both potential mechanisms whereby the content of atmospheric carbon dioxide could have been drawn down to promote cooling, on the assumption that this potential effect was not offset by increased addition of this volcanically derived greenhouse gas. Reduction in the supply of such carbon dioxide, with deposition of organic matter and silicate weathering continuing at the same rate, could also have produced cooling. A transient fall in temperature and increase in marine dissolved oxygen levels is well documented for OAE 2, in the form of the so-called Plenus Cold Event or Benthic Oxic Event, associated with southward invasion of certain boreal faunas and an increase in many redox-sensitive and/or chalcophilic elements in seawater caused by temporary loss of anoxic-euxinic sinks as basalt-seawater interaction continued apace. High-resolution studies of OAE 1a show at least three cooling episodes of probable global distribution, one of which (recorded in the stratigraphy of the so-called C4 Segment) is documented at high enough resolution to show correlation with an increase in carbon-isotope values that was probably due to a rise in the quantities of organic matter being buried globally, with consequent potential drawdown of atmospheric CO2 and/or reduction in volcanic input of this greenhouse gas. Both calcium- and lithium-isotope records suggest an increase in silicate weathering over the OAE 1a interval but current relatively low-resolution records cannot at present be tied to any one cooling episode, although the lowest Li-isotope values do derive from the C4 Segment. Evidence for reoxygenation of the ocean during the transient cooling episodes of OAE 1a is meagre, due to the lack of suitable sedimentary archives, although a negative sulfur-isotope excursion in a Pacific shallow-water carbonate section, which can be interpreted as due to oxidation of pyrite and/or sulfur-rich organic matter in the global ocean, suggests that this phenomenon may also have been a feature of the C4 Segment. Further work is required to elucidate the similarities and differences between OAE 2 and OAE 1a, but both phenomena are demonstrably global in reach and represent major disturbances to the carbon cycle with attendant effects on marine temperatures.
This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.
The composition and distribution of deep-sea sediments is the result of a multitude of climatic, biotic and oceanic conditions relating to biogeochemical cycles and environmental change. Here we ...utilize the extensive sediment archives of the International Ocean Discovery Program (IODP) and its predecessors to construct maps of deep-sea sediment type across two critical but contrasting boundaries in the Paleogene, one characterised by an interval of extreme warmth (Paleocene/Eocene) and the other by global cooling (Eocene/Oligocene). Ocean sediment distribution shows significant divergence both between the latest Paleocene and Paleocene Eocene Thermal Maximum (PETM), across the Eocene-Oligocene Transition (EOT), and in comparison to modern sediment distributions. Carbonate sedimentation in the latest Paleocene extends to high southern latitudes. Disappearance of carbonate sediments at the PETM is well documented and can be attributed to dissolution caused by significant ocean acidification as a result of carbon-cycle perturbation. Biosiliceous sediments are rare and it is posited that the reduced biogenic silica deposition at the equator is commensurate with an overall lack of equatorial upwelling in the early Paleogene ocean. In the Southern Ocean, we attribute the low in biosiliceous burial, to the warm deep water temperatures which would have impacted biogenic silica preservation. In the late Eocene, our sediment depositional maps record a tongue of radiolarian ooze in the eastern equatorial Pacific. Enhanced biosiliceous deposits in the late Eocene equatorial Pacific and South Atlantic are due to increased productivity and the spin-up of the oceans. Our compilation documents the enhanced global carbonate sedimentation in the early Oligocene, confirming that the drop in the carbonate compensation depth was global.
Abstract
Dissolved oxygen (O
2
) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been ...interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming
1,2
. However, models provide an unclear picture of future ODZ change in both the near term and the long term
3–6
. The paleoclimate record can help explore the possible range of ODZ changes in warmer-than-modern periods. Here we use foraminifera-bound nitrogen (N) isotopes to show that water-column denitrification in the eastern tropical North Pacific was greatly reduced during the Middle Miocene Climatic Optimum (MMCO) and the Early Eocene Climatic Optimum (EECO). Because denitrification is restricted to oxygen-poor waters, our results indicate that, in these two Cenozoic periods of sustained warmth, ODZs were contracted, not expanded. ODZ contraction may have arisen from a decrease in upwelling-fuelled biological productivity in the tropical Pacific, which would have reduced oxygen demand in the subsurface. Alternatively, invigoration of deep-water ventilation by the Southern Ocean may have weakened the ocean’s ‘biological carbon pump’, which would have increased deep-ocean oxygen. The mechanism at play would have determined whether the ODZ contractions occurred in step with the warming or took centuries or millennia to develop. Thus, although our results from the Cenozoic do not necessarily apply to the near-term future, they might imply that global warming may eventually cause ODZ contraction.
The naviculopsid morphology, which consists of an arch across the minor axis of a basal ring with two corners and sides, is the simplest of the Cenozoic silicoflagellate skeletal designs that include ...an apical structure. This skeletal design is most often associated with the genus Naviculopsis (geologic range: middle Paleocene to early Miocene), but also occurs among other silicoflagellate genera. This paper interprets the evolutionary history for silicoflagellates of naviculopsid morphology, beginning with a group of large two-sided silicoflagellates from the late early Paleocene, recently placed in genus Pseudonaviculopsis, and ending with a late Miocene group of naviculopsid morphologies associated with Bachmnnocena diodon subsp. nodosa. Between these is Naviculopsis, which has more than twenty species recognized in deep sea sediments.
A significant portion of the Naviculopsis taxa discussed in this paper are associated with plexus episodes of abundant and unusual variability from the late Paleocene, late Eocene and early Miocene. A possible explanation of the development of this skeletal morphology, as well as silicoflagellates with more complicated apical structures, is a need to decrease the size of portals, which may better support the cell boundary during mitosis.
•Silicoflagellates with naviculopsid skeletal morphology include Naviculopsis, Pseudonaviculopsis and Bachmannocena.•Naviculopsis diverged into two groups in late Paleocene, with plexid groups in each lineage.•The last prominent occurrence of naviculopsid silicoflagellate morphologies is within Bachmannocena diodon subsp. nodosa in the late Miocene.•The skeletal morphology may be an adaption to reduced portal size.
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